The present invention generally relates to automatic recording bias current setting apparatuses, and more particularly to an automatic recording bias current setting apparatus capable of automatically setting a most suitable recording bias current for a tape being used in a tape recorder which records and reproduces audio signals from the tape which has been arbitrarily selected from among different types of tapes.
If cassette tapes presently being marketed are generally classified, there are four types of cassette tapes, mainly, the normal type, chrome type, ferrichrome type, and the metal type cassette tapes. The magnetic characteristic of the same tape slightly differs according to the manufacturer. On the other hand, a tape selector is provided in a cassette deck for switching modes of the recording bias current, recording sensitivity, recording equalizer, and the like in three to four stages. The manufacturer adjusts the above recording bias current, recording sensitivity, recording equalizer, and the like, so that characteristics such as the frequency characteristic, distortion factor, signal-to-noise ratio, maximum output level (hereinafter simply referred to as MOL), and the like with respect to the tape being used becomes most suitable as a whole. Accordingly, when recording is to be carried out by using a tape other than that used by the manufacturer upon the above described adjustment, the recording cannot be carried out by utilizing the performance of the tape used to a maximum.
Hence, in order to use the tapes having various magnetic characteristics in a most suitable state, apparatuses have been developed conventionally for automatically performing the above various adjustments by use of a microcomputer, for example. These apparatuses mainly perform the adjustment of the recording bias current setting, adjustment of the recording sensitivity, and adjustment of the recording equalizer for mid and high frequencies, automatically and successively, for each of the above four type of tapes.
The adjustment of the recording bias current setting is particularly important. Internationally, it is recommended to record testing signals respectively having frequencies 333 Hz and 10 kHz onto a tape to be used by maintaining the recording level constant and gradually varying the respective bias currents, and set a bias current at a point where the difference between a MOL (maximum modulation level, or output level reproduced with a 3% distortion, and hereinafter simply referred to as a MML) of the testing signal of 333 Hz and a MOL (reproduced saturation output level, and hereinafter simply referred to as a SOL) of the testing signal of 10 kHz becomes equal to 12 dB when these testing signals are reproduced, as a most suitable bias current for that tape. However, the conventional apparatus does not employ two kinds of testing signals having frequencies of 333 Hz and 10 kHz, respectively. Further, in the conventional apparatus, the testing signal is recorded at a constant level even when the bias current is changed. Therefore, the bias current which was set as being the optimum value in the conventional apparatus, merely relied on the following three kinds of approximation.
One method generates the testing signal of 333 Hz from a testing signal generator within the reproducing apparatus, and records this testing signal onto the tape to be used by maintaining the recording level constant and gradually varying the bias current. The recorded testing signal is then reproduced, and a peak value is automatically detected. A bias current at a point when the peak value is detected, is set as the recording bias current. According to a second method, the testing signal of 333 Hz is recorded and reproduced as in the above first method. Then, a point where there is a reduction of 2.5 dB from the peak value is detected automatically, and a bias current at this detected point is set as the recording bias current. In a third method, the testing signals of 333 Hz and 6.3 kHz are both recorded and reproduced as in the above first and second methods, and a point where the reproduced level of the testing signal of 6.3 kHz is lower than the reproduced peak level of the testing signal of 333 Hz by 6 dB is automatically detected. A bias current at this detected point is accordingly set as the recording bias current.
However, the above conventional methods for setting the recording bias current are all approximation methods. Hence, there is an error between the recording bias current obtained, and the recording bias current at a point where the difference between the MOL (MML) of the testing signal of 333 Hz and the MOL (SOL) of the testing signal of 10 kHz becomes equal to 12 dB (the recording bias current set according to the internationally recommended method). Therefore, there was a disadvantage in that an accurate recording bias current could not be set.